Karen Oseland, of Eveleth, Minn., remembers peering inside the spinning class at her local YMCA and thinking, “That looks so hard.” Fast forward six years, and not only is Karen no longer an outsider, but she teaches the class 3 times a week.

“I love spinning—I love the boost of energy I get after I’m done!” she says of the stationary cycling class popularized in the 1990s.

Karen, now 51, had struggled with her weight since the birth of her daughter nearly 30 years ago. But the turning point came 20 years later, when her son, Gage, now 9, was born. Karen weighed nearly 300 pounds and was suffering from her third hernia. When her doctor broke the news that she’d need hernia surgery, he also suggested she consider gastric bypass.

“I didn’t want to do that. I know people can have complications, and the thought really scared me,” Karen says. “I started to think, I’m strong enough in spirit and in mind that I can do this without surgery.”

Karen’s first step was to visit a dietitian, who advised her to start writing down everything she ate. That helped her to avoid mindless eating and focus on making better choices, like whole grains and fruits and vegetables. She also learned proper portion sizes.

But the most dramatic change in her mindset was Karen’s approach to fitness.

“All my life I was a reluctant exerciser,” she says. “When I was a kid, I loved biking with a passion, but as an adult I had gotten away from that.”

Pleased with her dietary changes, but knowing that exercise would be key to getting even healthier, Karen joined the YMCA and started walking on the treadmill and swimming. Then, eventually, she got up the nerve to walk into that spinning class. As she’d imagined, it was a challenge, but she soon began building muscle tone and stamina, and day by day, it got easier.

Two years ago, Karen volunteered to teach the class when several of the instructors were out on maternity leave, and she’s been teaching ever since. She goes out of her way to make people feel comfortable, often remembering how afraid she was of a group fitness activity when she started.

“A lot of people are intimidated, and I try to let them know that they don’t have to do anything they don’t want to,” she says. “Spinning isn’t for everybody, and it’s a challenge for all of us.”

Karen’s popular with women her age, who enjoy seeing someone “like them” at the front of the classroom. But she did take some pleasure in impressing a younger student recently.

“My 29-year-old daughter came to my class, and she said, ‘Oh my God, Mom, I almost passed out!’ ” Karen says with a laugh.

While indoor cycling has become part of her routine, Karen, who was raised on a farm, couldn’t shake the memory of riding a bike through the country as a child, and how much joy it brought her. So in May, she splurged on a top-quality bicycle—and she’s already put 1,000 miles on it, thanks in part to a 50-mile bike ride she and a friend completed in August.

“When I get on my bike, I feel exhilarated — like I’m a kid again!” she says.

In all, Karen has lost 75 pounds and dropped 4 dress sizes. While she would like to lose more, she doesn’t obsess over the scale. Instead, she celebrates the strides she’s made in her health. She no longer has to take medication for high blood pressure, high cholesterol or type 2 diabetes.

What’s more, she has a new outlook on life. “I’m enjoying life, and I’m making new friends,” she says. “I’ve found that as you get older, you don’t have to stop.”